Variable electric resistance, potentiometer, and the like



Aug. 22, 1950 P. x. FOX 2,519,752

VARIABLE ELECTRIC RESISTANCE, POTENTIOMETER, AND THE LIKE Filed Nov. 22, 1949 //v vz/vro/e: PAM XA v/m Fox Patented Aug. 22, 1950 VARIABLE ELECTRIC RESISTANCE, TIOMETER,.AND THE LIKE POTEN- Paul X. Fox, Horsf'orth, near Leeds, England Application. November 22, 1949, Serialv N 0.. 128,819 In Great Britain November 26, 1948 1 Claim, 1.

This invention relates to variable electric resistances, potentiometers and the like and has for its chief object to provide an improved methodof' making contact with the resistance elements of such devices which will ensure an eflicient electrical contact with the resistance element at the selected point, without imposing any wear and tear on the. element as the adjustment is made.

In the variable electric resistance, potentiometer or the like according to the present invention, the resistance element is arranged spirally in, or on, a carrier or former and contact is made with the turns thereof by means of a roller which is axially slidable on a conducting rod adapted to be. carried around the axis of the former by means. of a suitable adjusting knob or the like, the arrangement. being. such that as the adjusting knob is rotated, said shaft is carried round the axis of the former to cause the roller to run along and make contact. with the spiral resistance element,

Conveniently, the resistance element is in the form of a. winding wound on an insulating mandrel which is adapted to sit in a spiral indentation in the carrier, or a sleeve therein.

In. order that. this invention may be the more clearly understood and. readily carried into efiect, reference may be made to the accompanying drawings which illustrate, by way of example, certain convenientembodiments of this invention and in. which;

Figure l is a longitudinal section through one form of potentiometer according to the present invention, and

Figure 2 is a transverse section through the same.

Figure 3 is: a perspective view of the roller and carrier. assembly.

Figure 4 is an end. elevation of the potentiometer, and

Figure 5 is a transverse section on line 55 of Figure 1, illustrating the means for stopping the movement of the roller at, or near, the end of the resistance element.

Figure 6 is a longitudinal section through a modified form of potentiometer according to the present invention, and

Figure 7 is a local section through one end of the roller and carrier assembly showing the man ner in which the stop is effected.

Referring now more particularly to Figures 1 to 5 of the accompanying drawings, the potentiometer therein illustrated comprises an outer cylindrical casing I!) closed at both ends by insulating discs H, which are appropriately secured in the ends of the casing. The winding of the potentiometer comprises a mandrel l2 having a resistance wire :3 wound thereon. The man drel l2 may be made of any suitable material but its surface, at least, must be insulating. Thus, it is convenient to form the mandrel- 2 of copper wire the surface of which is coated with insu-- lating material but it may equally well be made wholly of insulating material. The wound resistance element is disposed in a spiral form and sits in a spiral indentation M in a sleeve l5 suitably secured within the casing iii. For the sake of simplicity of manufacture, the sleeve I5 ispreferably longitudinally split into two parts, as will be seen on reference to Figure 2. The discs H are shouldered to take into the ends of the casing it and they form the means of locating the two-part sleeve IS in position.

Freely rotatable within the end disc l i and axially of the cylindrical casing 10 is a shaft :6 which is provided externally of the casing with a manually operable knob IT, or with any other desired. means of adjustment. Inside the casing L0,. the shaft I6 is provided with two moulded discs or arms l8 which may be pinned or otherwise secured to the shaft and which are disposed.

one at one end of the casing Ill and the other towards the other end of the casing H] as will be appreciated from. Figure l. The shaft and arm assembly is spring-biased, preferably to wards the front end of the casing, by means of a compression spring [9 disposed between the end arm [8 and the juxtaposed end disc H. A

1 rod 28 extends between the two internal arms,

I18 eccentrically of the operating shaft l6, its ends being arranged in radial slots 2! in. the armsl8. On the said rod 20 freely rotatable a. contact roller 22, the arrangement of the parts being such that the roller 2:2 contacts. with. the spiral resistance element within the casing, whereby as the operating shaft i5 is rotated, the roller 22: is carried round inside the casing" by reason of the eccentric mounting of the rod 29 on which it is mounted, and so runs continuously round and along the spiral resistance element. The roller 22 is sprung against the resistance element by means of two wire springs 23, one on each end, which springs are connected between the arms l8 and the rod 20.

By this means, it will be seen that contact may be made with any point of the spiral resistance element at will, and that no wear of the resistance element will take place since there is no relative movement between the roller and the point of the resistance element with which it is in contact when adjustment takes place.

In order to provide a stop for the roller at the end of the resistance element, lugs 24 are moulded into the sleeve at the end of the resistance element, these lugs extending into the path of the roller and forming positive stops therefor.

If there is a tendency when the roller runs on to this stop for it to open-circuit the potentiometer, this may be overcome by means of a small piece of silver or like foil which is ended off on the end of the winding.

The two ends of the resistance element are taken out to contacts 25 on the rear end of the potentiometer, and in order to make contact with the roller, advantage is taken of the fact that there will be continuous rubbing contact between a washer 21 disposed about the bearing for the operating shaft H in the end disc and a washer 28 on the back of the rear arm due to the presence of the spring l9. Connection is made through the biasing wire '23 to a stud 29 which is connected with the washer 21. Adequate rubbing contact to the roller is by this means obtained at all times. I

In a modified form of potentiometer, illustrated in Figures 6 and 7, the two moulded arms l8 of the construction shown in Figures 1 to 5, are replaced by two Phosphor-bronze, or like, springs 30, which are bent in towards one another as will be seen from Figure 6. These Phosphor-bronze springs 30 carry the roller 22 and its associated rod 26 in the same way as the moulded arms [8. The inward inclination of the'Phosphor-bronze springs 39 is so adjusted that the roller runs into contact with the same (at one end, of course) just prior to the roller reaching the end of the resistance element, see the full line position of the parts in Figure 7. A further fraction of a turn of the operating shaft 16 in driving the roller farther along the resistance element forces the Phosphor-bronze spring 30 outwardly, whereupon the rod 29 carrying the roller 22 is'moved axially and caused to engage with a positive stop 3| moulded into the end disc ll of the potentiometer (see the broken line position of Figure 7).

The preferred form of roller used in carrying out the present invention comprises a disc 40 of conducting material integral with, or carried by a metallic sleeve M which is in good sliding contact with the rod 20. Side cheeks 42 of insulating material are mounted on the sleeve 4i and the ends of the said sleeve 4| are riveted, or

splayed over to hold the discs 42, one on each side of the conducting disc 40. The insulating discs 42 are of a slightly greater diameter than the conducting disc 40 and thus form flanges which are disposed one on each side of the spiral resistance element, these insulating flanges serving to guide the roller along the spiral resistance element and ensuring that good contact will be made at all times.

Number Name 2,361,010 Cary et a1. Oct. 24, 1944- 2,454,986 Beckman Nov. 30, 1948 2,473,048 Beckman June 14, 1949 FOREIGN PATENTS Number Country Date In a further modified form of the present invention, the helical potentiometer or variable electric resistance incorporates a single or multiple-start screw-thread which is of the same pitch as the winding of the resistance element, and is held against movement in one end of the potentiometer or the like. A nut on the said screwthread carries a roller type contact and is held against rotation so that on rotation of the screw thread and the resistance element as a unit, the nut will move up and down the screw thread to efiect contact with the helical resistance element. The roller may be of the form described in Patent Specification No. 582,476, or alternatively, a precious metal contact may be used.

In a further modified form of the present invention, the wire wound resistance element hereinbefore described is replaced by a sprayed, or otherwise provided, carbon or like film. Thus, for example, it is possible to deposit a spiral track of the pyrolytic type of high stability carbon on a former and to use the same as the spiral resistance element. Alternatively it is possible to spray a coating of such material on the inside of the former and then to cut or machine the surface to form the helix.

I claim:

A variable electric resistance of the type in which a spirally arranged winding is provided with a contact-making roller engaging predetermined points of said winding, said roller being axially slidable on a rod, formed of electrically conductive material, and including means for moving said rod in a path eccentric to said winding, whereby said roller is made to traverse said winding longitudinally, characterized in that, for the support of said conductive rod, there are provided, near the respective extremities thereof, two arms formed of springy material and so bent in towards one another that said roller shall contact the corresponding arm just prior to reaching a corresponding end of said winding, and positive stops located stationary relative to said winding, adjacent the respective ends of said rod, whereby rotation of said rod in the respective directions ultimately causes said roller to flex a respective springy arm and thereby to cause said rod to engage said Positive stop.

PAUL X. FOX.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Date Great Britain Nov. 18, 1946 

